1. Field of the Invention
The present invention relates, in general, to a method of manufacturing a piston for variable displacement swash plate compressors of air conditioning systems and, more particularly, to a method of manufacturing a hollow piston for such compressors, the hollow piston being light enough to receive a low inertia force and being capable of improving the fluid compressing efficiency of a compressor.
2. Description of the Prior Art
As well known to those skilled in the art, a conventional compressor for air conditioning systems of automobiles selectively receives engine power transmitted from a pulley. Such a selective transmission of the engine power to the compressor is accomplished by an electromagnetic clutch. Upon receiving the engine power, the compressor compresses gas refrigerant flowing from an evaporator, thus making high temperature and high pressure gaseous refrigerant prior to feeding the refrigerant to a condenser. In such a case, the high temperature and high pressure gaseous refrigerant from the compressor is easily liquidized in the condenser.
An example of such compressors is a swash plate compressor. In a conventional swash plate compressor, a wobble plate is inclinedly mounted to a drive shaft cooperating with an engine, thus being rotated by engine power. A plurality of pistons are coupled to the outside edge of the swash plate with a shoe being interposed between the swash plate and each piston. When the swash plate is rotated by the engine power, the above pistons rectilinearly reciprocate within the bores of a cylinder, thus sucking, compressing and exhausting gaseous refrigerant.
The object of the present invention is to provide a method of manufacturing a hollow piston for variable displacement swash plate compressors of air conditioning systems, which produces a desired hollow piston by separately forming a body and a cap and by simply machining the body and the cap prior to simply welding the body and the cap together into a single structure through a frictional welding process in atmospheric air at room temperature, and which thus reduces the defective proportion of resulting pistons, remarkably reduces the production cost of the pistons due to a reduction in both the equipment cost and the processing cost, and allows the pistons from the welding process to be free from micro pores, thus improving the refrigerant compressing efficiency of a compressor.
In order to accomplish the above object, the present invention provides a method of manufacturing a hollow piston for variable displacement swash plate compressors, comprising: a forging step of separately forming a body and a cap, the body consisting of a bridge part, with a hollow cylindrical head part and a grip part being formed at both sides of the bridge part, and the cap consisting of a hollow cylindrical head part and a grip part formed at both sides thereof; a pre-welding machining step of cutting end surfaces of the head parts of both the body and the cap, thus forming junction surfaces at the end surfaces of the head parts; a frictional welding step of integrating the body and the cap into a piston member by frictionally welding the junction surfaces together while forcing the body and the cap toward each other with the body and the cap being held by jigs at their grip parts; a post-welding machining step of cutting a side surface of the piston member while holding the piston member using a jig, thus primarily forming a side surface of a piston head; a coating step of forming an abrasion resistance layer on the side surface of the piston head; a polishing step of polishing the side surface of the piston head, thus finally forming a desired side surface of the piston head; a recess cutting step of cutting the bridge part of the body of the piston member at a predetermined portion, thus forming a swash plate seating recess with a shoe pocket, the shoe pocket being formed by cutting a sidewall of the swash plate seating recess at diametrically opposite portions so as to form concave surfaces at the diametrically opposite portions; and a final cutting step of removing the grip parts from both ends of the piston member, thus forming front and rear end surfaces of a resulting piston.
In the method of this invention, a desired hollow piston is produced by separately forming a body and a cap through a die-casting process or a forging process and by simply machining the body and the cap prior to simply welding the body and the cap together into a single structure through a frictional welding process under atmospheric air at room temperature. The method of this invention thus remarkably simplifies the pre-welding machining process and welds the body and the cap together into a single structure without requiring a high processing precision different from a conventional method. Since the welding process of this invention is simplified, it is possible to reduce the defective proportion of resulting hollow pistons in addition to a reduction in both the equipment cost and the processing cost while producing the hollow pistons. This finally reduces the production cost of the hollow pistons. In addition, the frictional welding process of this invention is free from forming micro pores in the resulting pistons, and so the pistons of this invention do not reduce the refrigerant compressing efficiency of variable displacement swashplate compressors.